There are innumerable industries wherein milling is a necessary operation to refine a product to the point where it can be usefully employed. The products involved have ranged from rubber materials to foodstuffs to paints and enamels. More recently, the emulsification and homogenization of slurries of water, fuel oils and coal dust or particles especially for use as low cost fuel for jet injection in power plants has become an important application for milling equipment. In these and other applications the material to be milled or refined is in the form of a slurry containing solid particles and it is desired to reduce the size of the particles and to emulsify and disperse them throughout the product.
An early and still common type of mill is the so-called ball or pebble mill in which the product to be refined is placed in a container with balls or pebbles and the container is rotated on a rack in such a manner that the balls or pebbles crush and mix the solid particles. The process is notoriously slow and energy consumption in turning the mill is not only excessively high, it is largely wasted because as much as 75% of the energy is utilized in turning the balls or pebbles. Moreover, for obvious reasons, ball and pebble mills cannot be used with adhesive products.
Another common mill is the roller mill used in the production of paint and similar products. Often, sets of three, or sets of five rollers are employed and the material is simply spread upon the rollers and ground between them as they rotate. These machines are objectionable not only because of their great cost and the cumbersome nature of the structures but also because the material being treated is exposed to the atmosphere with attendant dangers to the operator and to the environment. In some instances, depending upon the product being treated, elaborate precautions must be taken to avoid operator injury and air pollution.
Conventional colloid mills avoid many of the drawbacks associated with ball mills and roller mills. These mills generally include an enclosure in which a rotor turns relative to a stator and slurry passes between these elements. The rotor smears a thin film of the slurry on the cooperating stator face and there is a hydraulic shear effect upon the product which can be accentuated by increasing rotor speed. However, with increased speed, slippage of the slurry and excess power consumption occur. Also, in many common slurries such as resins, oils or adhesive products, heat is generated because of the high rotor speed. It is often attempted to avoid such problems either by increasing the gap between the rotor and stator or by cooling the rotor, the stator, or both of them.
However, heat transfer between product and apparatus is not at all efficient and cooling of the apparatus is relatively ineffective in reducing temperature. Opening the gap between the rotor and stator is equally unavailing because the slurry passes through without sufficient refining taking place. Finally, if speed of the rotor is reduced, less heat may be generated but only at the cost of loss of work on the product because the smearing and shear effects are lessened and the particles in the slurry remain inadequately refined.
By way of specific example, if a paint film were being processed, the output product would have a texture comparable to sandpaper where the requirements are for a paint having a texture comparable to a glass surface.